1. Field of the Invention
The present invention relates to a method for producing a solid polylactic acid having a high molecular weight. The polylactic acid obtained by the method of the present invention has a high molecular weight, which may take various shapes such as granular, pelletized, and plate-like forms.
2. Discussion of the Related Art
A polylactic acid is a biologically very safe polymer, and its degradative product, namely lactic acid, is absorbed in vivo. Having the above properties, the polylactic acid is used for medical purposes, including surgical sutures, sustained-release capsules in drug delivery systems, and reinforcing materials for bone fractures. Moreover, it is noted as a degradable plastic, because it degrades under natural environmental conditions. It is also widely used for monoaxially and biaxially stretched films, fibers, extrusion products, and various other purposes.
The methods for producing a polylactic acid may be as follows: In one method, lactic acid is directly subject to dehydration condensation to give a desired product. In another method, a cyclic lactide used as a dimer, is first synthesized from lactic acids and then purified by such methods as crystallization, followed by ring-opening polymerization. Various procedures for synthesizing, purifying and polymerizing lactides are disclosed in U.S. Pat. No. 4,057,537, EP-A-261,572, Polymer Bulletin, 14, pp. 491-495 (1985), Makromol. Chem., 187, 1611-1629 (1986), and other chemical literatures. Also, JP-B-56-14688 discloses a method for producing a polylactic acid comprising polymerizing a bimolecular cyclic diester, as an intermediate, using tin octylate or lauryl alcohol as a catalyst. The polylactic acid thus obtained is pelletized into various forms, such as spheres, cubes, columns, and disrupted pieces, in a size of from a rice grain to a bean, to facilitate its handling in the forming process.
However, the polylactic acid having a high molecular weight of from 100,000 to 500,000 has a high melting point of from 175.degree.-200.degree. C. Conventionally, when the final polymer product of the above polylactic acid is taken out from the reactor in a molten state and heated to a temperature higher than its melting point, the polylactic acid undergoes decomposition and coloration. Moreover, a large amount of lactides remain in the polymer by heating at the above temperature, presumably due to the fact that a polymer-lactide equilibrium is shifted toward the lactide side at the above temperature.
These lactides and decomposition products thereof are likely to sublime during injection molding or spinning of the polylactic acid pellets used as a starting material and undesirably adhere to dice or nozzles, and thereby the operation is hampered. Furthermore, the lactides and the decomposition products lower the glass transition temperature and the melt viscosity of the polymer, resulting in drastic deterioration of formability and thermal stability.